Purification device for alkali metal cells



Nov. 13, 1956 A. FENTRESS 2,770,592

PURIFICATION DEVICE FOR ALKALI METAL CELLS Filed NOV. 6, 1953 3 Sheets-Sheet 1 FIG. I

t INVENTOR.

ARTHUR FENTRESS BYM ATTORNEY 13, 1956 A. L. FENTRESS 2,770,592

PURIFICATION DEVICE FOR ALKALI METAL CELLS Filed Nov. 6, 1953. 3 Sheets-Sheet 2 INVENTOR.

ARTHUR L. FENTRESS ATTORNEY av. 13, 1956 A. L. FENTRESS 2,770,592

PURIFICATION DEVICE FOR ALKALI METAL CELLS Filed Nov. 6, 1953 SSheets-Sheet 5 FIG. 5 FIG. 6

INVEN TOR. ARTHUR L. FENTRESS BYZMW ATTORNEY United States Patent PURIFICATIDN DEVICE FOR ALKALI METAL CELLS Arthur L. Fentress, Baton Rouge, La, assignor to Ethyl Corporation, New York, N. Y., a corporation cf Delaware Application November 6, 1953, Serial No. 390,543

Claims. (Cl. 204-245) This invention relates to an apparatus for the production of alkali metals by the electrolysis of fused salts. The invention more specifically pertains to improved apparatus for insertion in or addition to cells of the above type which are used for producing sodium or similar alkali metals by the electrolysis of fused alkali chlorides.

The manufacture of sodium and other alkali metals is carried out in cells which are basically similar to the Downs cell described in U. S. Patent 1,501,756. The characteristic of the Downs cell is formation of sodium and chlorine in an annular zone between a centrally mounted anode and a surrounding cylindrical cathode. The fused electrolyte or bath employed in such apparatus is the chloride of the desired alkali metal but includes substantial quantities of calcium chloride. The calcium chloride is present in sufficient quantities to provide a reasonably low melting point of the fused mixture, but not in such high proportions that the electrolysis provides large amounts of calcium metal concurrently. Ideally, of course, the bath composition would be adjusted so that no calcium would be formed whatsoever. However, this ideal situation is unobtainable as a practical matter. As a result, the catholyte sodium metal usually is approximately saturated with calcium metal at the temperature of operation.

The cathode or metal product from electrolyte cells is discharged by slow flow upwardly through a collector and discharge pipe, usually termed the riser pipe, to a discharge line. The movement is caused by the hydraulic head resulting from differences in densities of th fused bath and the metal product.

Because of the large differences in temperature of sodium, plus the great variation of calcium solubility in sodium with respect to temperature, the calcium appears as a solid contaminant in the sodium product. This impurity problem has long been recognized. Hence, specific purification operations, such as filtering, were em ployed to obtain a high quality sodium product. Improvement in this situation, that is, in the quality of sodium discharged from an operating cell has been effected by utilizing the riser pipe as a purifying and separating device. As already noted, the sodium product is discharged from a cell by upward flow through a vertical pipe, or riser pipe under the influence of hydrostatic head owing to the difference in density of the sodium product and the fused salt electrolyte within the cell. This riser pipe extends for some distance above the upper confines of the cell so that a portion is extending into the atmosphere. This portion of the riser pipe can be employed as a heat transferring surface to remove heat from the outwardly flowing stream of sodium product. This results in temperature reduction which in turn results in solidification of an appreciable portion of the calcium impurity, this being deposited upon the wall of the riser pipe. In order to prevent such deposition from clogging up eventually the riser pipe, or effectively blanketing the riser pipe wall and preventing further heat removal there- 2,770,592 Patented Nov. '13, 1956 ICC through, means have heretofore been employed for dislodging the solidified calcium particles from the riser pipe wall. These means, customarily referred to as ticklers, comprise usually a plurality of blades corresponding in length generally to about the diameter of the riser pipe, and afiixed to a shaft which is normally concentric with the riser pipe, extends through the top of the pipe, and is capable of rotation and limited vertical movement.

In operation of a cell, this above described tickler is rotated and lifted and pushed downward a number of times, at various periods or cycles of intervals of about one-half hours. The tickler has provided a highly effective improvement in cell operations when operated consistently, according to a definite schedule, and thoroughly upon each operation cycle. There is a tendency however, in all apparatus relying upon cyclic manual operation, for inconsistencies to appear owing to normal human variability. This is particularly evident during the summer months, because the necessity of working over operating cells maintained at a temperature of the order of 500 C. naturally introduces reluctance on the part of operators to adhere rigorously to desired schedules. If operating cycles are curtailed or a particular operation is omitted from the scheduled time, the temperature of a riser pipe will of course rise, the amount of impurity carried over will increase, and because of the increase in temperatures the tickler proper is subject to warpage and degradation because of the high temperature of the upflowing stream.

It is therefore apparent that the prior apparatus leaves much to be desired in the operation and effectiveness in a large number of sodium cells. An object of my invention accordingly is to provide apparatus eliminating the need for manual operations of this sort. to provide product purity. A further object is to provide an apparatus capable of extensive continuous operation Without disassembly as required by the prior method. A further object is to provide apparatus for more effective cleaning of the riser pipe walls than has heretofore been possible. Other objects will appear hereinafter.

The apparatus comprises generally the combination, with a cell riser pipe, of apparatus having a helical element within and substantially coextensive with the riser pipe, said helical element being operatively connected to means for providing rotatable motion and intermittent vertical motion. The helical element approaches in diameter the interior diameter of the riser pipe so that upon rotation in the appropriate direction, calcium particles are dislodged from the riser pipe walls and given a downward thrust component. The vertical movement referred to is a reciprocating motion, the upward! movement being a slow movement, at a rate lower than the downward lead of the helical element. The downward movement required is of course equal in magnitude to the upward movement, but instead of being a gradual movement, is a sharp thrust for purposes hereinafter described.

The apparatus is capable of assuming a variety of forms, which will be readily understood from the detailed description below and the accompanying figures. The figures illustrate several of the numerous embodiments of the apparatus, all having the basic elements as above described. The figures include Figure 1 which is a schematic elevation of an electrolytic cell showing the position of an embodiment of the apparatus operatively positioned therein. Figure 2 presents, in more detail, a partially sectioned elevation of a cell riser pipe having an embodiment of the invention within. Figure 3 illus trates an alternative means for providing the rotative and vertical motions heretofore referred to. Figure 4 illustrates construction details of a preferred form of support structure for the helical element. Figure 5 is a plan view of the. support structure. Figure 6 shows a cross sectional view of a member which is a portion of the support and actuating structure shown by Figure 4.

Referring to Figure 1, a typical cell is shown in cross sectional elevation. The cell includes a cell wall and bottom 13., an anode 21, a cathode casting 31, a chlorine collector dome 41, and a discharge conduit or riser pipe assembly 51 for discharging the metal cathode product. The interior of the cell is filled with a fused bath 12 of metal chlorides. Current is passed through the bath in the annular space 22 defined by the anode 21 and the cathode 31. A diaphragm 32 of woven metal positioned within the annular space 22 facilitates and continues segregation of the sodium cathode product released at the face of the cathode 31 and the chlorine released at the anode surface. Both of these products rise in the fused liquid bath. The chlorine is collected by the chlorine dome 41 and transmitted to Subsequent operations by the discharge line 42. The sodium is collected in an annular trough 23, the highest point thereof being adjacent the entrance 52 to the riser pipe proper. The dimensions of the cell and the depth of the fused bath above the entrance 52 to the riser pipe are such that the dilferent densities provide flow of the sodium metal to the relatively elevated point exterior of the cell at which the riser pipe discharges to a collector hopper 53. By appropriate proportions of these dimensions and the height of the riser pipe 51 outside the confines of the cell top 13, the flow path of the sodium is such that the sodium can be effectively cooled to a temperature such that a high degree of precipitation of calcium metal contaminant is effected.

The heat removal for this cooling operation is facilitated by fins 54 attached to this portion of the riser pipe.

Within the riser pipe 52 is shown the helical element 71 of the present invention, illustrated more specifically by Figure 2. Operating mechanism for imparting the desired movement to the helical element 71 surmounts the riser pipe 52 and is surrounded by a sheet metal enclosure 85.

Figure 2 shOWs a typical embodiment of the invention situated within a riser pipe but shown on a larger scale than in Figure 1. In the embodiment shown in Figure 2, the helical dislodging element is a continuous spiral blade 71. The spiral'blade is attached to a shaft 72, which has a square section 73 at its uppermost end. This square section engages a matching hole in the plate 74 thereby permitting vertical movement of the spiral-shaft assembly. The plate 74 is a part of a cage assembly 77 which is attached to a driving shaft 79.

The spiral blade 71 has a diameter approximately equal to that of the riser pipe 52 to which it is mounted. The spiral blade 71 is attached to the shaft 72 and supported therefrom by spokes or arms not shown in Figure 2. The riser pipe is connected to a discharge line 55 near its upper end. A cover plate 56 has attached to it a bearing member 75 serving as a bearing for the shaft 72 and also functioning as a cylindrical cam. A pin 76 through the shaft 72 of the helical blade shaft assembly rides on the cylindrical cam 75.

In operation, the spiral blade-shaft assembly is rotated at a moderate or low speed, the blades disloding and cutting calcium solids from the walls of the riser pipe 52. It would be expected that this action would be fully adequate inasmuch as the calcium particles have greater density (about 1.55) than the liquid sodium (density=l) in the riser pipe. However, apparently these particles are sufiiciently near the melting point to be very strongly adherent to the metal surfaces. There is a definite tendency to stick to the moving parts after removal from the walls. The vertical movement provided by the cam 75 fully mitigates this tendency. The cam 75 provides for a sharp vertical drop every revolution of the spiral blade 71. It should be noted that in this embodiment that the vertical movement is a sharp drop rather than a mere acceleration in a downward direction. This downward plunge results in a secondary dislodging of the calcium particles from adherence to the surfaces of the blade-Shaft assembly, so that they will descend into the cell interior.

The pitch of the helical element is not critical inasmuch as variations in pitch can be compensated for by variations in the speed of the rotation thereof. However, it has been found that too great a pitch does not provide as effective a downward thrust component in dislodging the calcium crystals. On the other hand, too small a pitch has the effect of blanketing the surface of the riser pipe to a certain extent and interfering with the precipitation of the calcium particles thereon.

Accordingly, in all cases it is preferred that the pitch of the helical element be in the range of one-half to twothirds of the interior diameter of the riser pipe. Thus, in a riser pipe of 8 inches diameter a helical pitch of at least 4 inches is desired, and having a 9 inch riser pipe, the pitch should not exceed 6 inches. The helical blade shown in all embodiments illustrated is a single flight blade but the principal of the invention is not confined to this form. Thus, two or even three blades could be used, but a multiplicity of blades suffers from the same disadvantage as the single type blade having too small a pitch.

The effective downward thrust of the helical blade or blades is of course a concurrent function of the speed of rotation as noted above. In certain forms of the apparatus, speed of rotation does not affect appreciably the effectiveness of discharge as will be described hereafter. However, in the common forms of the invention, specifically shown by the figures, support structure is used including a shaft (for example shaft 72 in Figure 2) with spokes projecting from the shaft and afiixed to the helical blade for support and alignment thereof. In the rotation of such embodiments, the movement of the spokes has a tendency to disturb the upward movement of the sodium stream. This can be partly compensated for as described hereafter, but too high a speed of rotation unduly agitates this stream and minimizes the effectiveness of the riser pipe for purification.

Considering suitable support structure, reference is made to Figures 4 and 5, Figure 4 being an elevated view of the support structure incorporated in the embodiment shown in Figure 2. Figure 5 is a plan view of this structure also showing a portion of the helical blade. Turning to Figure 4, shaft 72 is provided with a plurality of spokes 78. The spokes extend to the blade 71 and are welded to the interior surface thereof. The spokes are arranged in a spiral pattern opposite in pitch to the pitch of the blade 71. .The pitch of the spoke arrangement is variable but generally corresponds to that providing approximately equivalent upward rate movement corresponding to the natural flow of the sodium stream. Thus, in a cell wherein the sodium is discharged at the upward rate of approximately one inch per minute in the riser pipe, in a shaft blade assembly at a speed of about two revolutions per minute the blades would follow a spiral pattern having an upward lead of approximately 11". The radial distribution of the spokes is not particularly important and is discussed hereafter. Too many spokes would be disadvantageous. It is preferred that spokes be distributed at a radial space of at least and usually of the order of 210 to 270". It will be understood that the radial distribution can be affected by the relation of the pitch of the spiral pattern in which they are aranged to the pitch of the spiral blade. As already indicated, it is desirable that the rotation of the assembly should not result in disturbing agitation of the upward stream and to this end the spokes may be relatively flat in cross section, their longer cross sectional axis pitched at an angle roughly corresponding to the upward flow. As another example in addition to the example mentioned above, for the approximate upward flow of 1 to 4 inches per minute, with the spiral blade rotating at 2 revolutions per minute, in an 8 inch diameter riser pipe, the spokes are tilted at from 8 to 16 to the horizontal.

In embodiments wherein a relatively high pitch of the spiral blade is employed and the speed of rotation is quite low, the necessity of desirability of using spokes of flat strip or similar configuration is eliminated. In such instances, the spoke can advantageously be of a cross section to facilitate dislodging of adherent calcium particles from its surface upon downward thrust. A preferred form of spokes for such instances is illustrated by Figure 6, showing a single spoke in cross section affixed to the shaft. The spoke 78 in this embodiment has a diamond cross section, with the long axis of the diamond in a vertical direction. This configuration assures the least possible adherence of calcium particles to the spoke surfaces inasmuch as there are no upwardly directed horizontal surfaces.

As already indicated, a rigid support structure is not absolutely essential. In fact, in some instances the spiral blade may be entirely self-supported and the actuating shaft may be connected only to the topmost flight of the spiral blade. In such embodiments of course the spiral blade must have a more rigid cross section because of the relatively long vertical distance wherein it operates. In the operation of such embodiments a supplemental benefit is accrued due to the absence of support structure. A spiral blade having a resemblance to a spring is capable of deformation which is advantageous for several reasons. For example, if a particularly hard or large deposit occurs on the riser pipe wall, the helical blade can deform either vertically or laterally. The vertical compression would provide a steadily increasing downward cutting thrust, and if this accentuated force is not sufiicient to dislodge the deposit the absence of a rigid control structure will permit the spiral to deform laterally and pass over the deposit in that revolution.

As previously mentioned, various alternative means for providing the desired downward thrust can be utilized, of which the embodiment shown in Figure 2 is only one example. The embodiment of Figure 2 provides a down- Ward thrust at the same point in each single rotation. In some instances, it is very desirable to provide the downward thrust at a variable point in the circular pattern. This, for example, is advantageous with respect to clearing the opening through which the sodium enters the discharge pipe 55. This is particularly true in those embodiments where the downward thrust is appreciably less in diameter than the discharge opening. Alternative means for providing this variable locus of the downward thrust is illustrated by Figure 3. Referring to Figure 3, shaft 72 is fitted with a disc 81, journalled on the shaft and retained by collars 84. The disc normally rests on a cam 82. Cam 82 is rotated by its support shaft 83, except when the shaft and the disc is at the lowermost point of its vertical travel, at which time the collar 84 is supported by the face of the bearing 57. This structure prevents undue shock on the cam 82. Cam 82 is rotated at a speed which provides a vertical drop at a variable point in the revolution of the shaft and the disc 81. Thus, it is preferred that when the shaft is rotated at a speed of, for example, 3% revolutions per minute that the cam rotate at a speed of 4 revolutions per minute. Thus, the vertical drop will be provided by the portion 85 of the cam every 337 of the rotation of the blade and shaft. This variation has several desirable effects. It specifically assures that a consistent pattern in the downward thrust will be avoided. It will be apparent that alternative mechanisms may be provided to accomplish this variable drop-thrust effect and also to provide a supplemental action which is particularly beneficial. This supplemental action involves applying a twisting moment to the shaft and blade assembly which tends to force the blade to one side of the riser pipe. In order to utilize such a twisting movement and supplemental wiping action, provision must be made for said twisting moment, so that it is varied in position around the center of the riser pipe. This may be ac: complished in several ways. For example, a planetary gearing arrangement may be used in combination with a cylindrical cam such as the cam 75 in Figure 2, so that the said cam is concurrently rotated about its vertical axis but at an appreciably different speed than the blade and shaft assembly. Owing to the suspension of the blade and shaft assembly by a pin projecting from the shaft and bearing on the cam, a relatively small but appreciable moment is applied to the shaft tending to force the blade toward the wall of the riser pipe which is below the pin. By providing for rotation of the cam as indicated, this particular action is continuously moved around the internal periphery of the riser pipe. In order to take advantage of this twisting moment, it is desirable that the hole in the cover plate 56 should be a relatively loose fit thereby permitting some twisting moment. Other means of providing the vertical movement of the blade and shaft assembly and of the twisting; moment, if desired, will be apparent to those skilled in the art.

As an illustration of the effectiveness with which the apparatus accomplishes the objectives of the invention, in a typical cell apparatus as shown in Figure 2 was employed for a period of three days. At: the termination of that time, it was removed, solely for inspection, from the riser pipe. The blade and shaft were found virtually free of any adherent metallic particles and the riser pipe walls were absolutely free. In contrast, when employing a manually agitated tickler according to prior practices, ticklers were necessarily removed after an average three day period because they are no longer operable manually. This removal was necessarily accomplished by pulling wit-h an overhead crane because of the large accumulation of calcium on the walls and parts of the tickler.

Having fully described the apparatus of my invention and the manner of best utilization, what I claim is:

1. In a cell for electrolytic production of sodium from fused salts, having a riser pipe extending above the cell for concurrently discharging the sodium metal, cooling and precipitating calcium impurity and returning the calcium metal, the improvement comprising a helical blade within the riser pipe, the helix having a length approximately equal to the length of the riser pipe and a diameter approaching the diameter of the riser pipe, and being positioned within said riser pipe, the blade being relatively narrow with respect to the diameter of the riser pipe thereby defining an open space within said helix, said rhelical blade being attached to a shaft concentric with and projecting from the top of the riser pipe, the blade and shaft being rotatable, and vertically movable in a limited distance, motor means operatively corinected to the shaft for rotating the shaft and helical blade, and means synchronized with said motor means for imparting a vertical reciprocating motion to the shaft and helical blade at least once in every revolution, the upward portion of the reciprocating movement being at a low and substantially uniform rate, the downward portion being abrupt and accelerated, thereby dislodging calcium particles from the riser pipe and preventing adhesion of said calcium particles to the helical blade.

2. The apparatus of claim 1 further defined in that the helical blade has a helical pitch of from one-half to twothirds of the riser pipe diameter, and the: means for reciprocating vertical movement provide movement in a vertical path less than the pitch of the helical blade.

3. The apparatus of claim 2 further defined in that the helical blade is supported solely by a connection from the upper portion of the blade to the shaft, for transmitting rotating and reciprocating motion.

4. The apparatus of claim 2 further defined in that the shaft extends through the full length of the helical blade and provides support therefor and is connected to the blade by plurality of spokes having a thin rectangular cross section, said spokes being aligned in a spiral having a reverse pitch to the pitch of the helical blade and spaced radially at intervals of from 210 to 270, the long cross sectional dimension of said spokes being inclined at an angle of from about 8 to 16 from the horizontal, and the means for providing reciprocating vertical movement providing a complete vertical movement cycle at uniform intervals corresponding to rotation of the shaft through a non-integral number of revolutions.

5. The apparatus of claim 2 further defined in that the shaft extends through the full length of the helical blade and provides support therefor and is connected to the blade by a plurality of spokes, said spokes having a uniform radial spacing of from 210 to 270, and each spoke having *a diamond cross section, the long dimension of the diamond being vertical.

References Cited in the file of, this patent UNITED STATES PATENTS 902,315 McCarty Oct. 27, 1908 1,932,068 Engleoright Oct. 24, 1933 2,068,681 Hulse Jan. 26, 1937 2,182,680 Ruggs Dec. 5, 1939 2,205,757 Wheat June 25, 1940 2,390,115 McNitt Dec. 4, 1945 FOREIGN PATENTS 748,684 France Apr. 25, 1933 

1. IN A CELL FOR ELECFROLYTIC PRODUCTION OF SODIUM FROM FUSED SALTS, HAVING A RISER PIPE EXTENDING ABOVE THE CELL FOR CONCURRENTLY DISCHARGING THE SODIUM METAL, COOLING AND PRECIPITATING CALCIUM IMPURITY AND RETURNING THE CALCIUM METAL, THE IMPROVEMENT COMPRISING A HELICAL BLADE WITHIN THE RISER PIPE, THE HELIX HAVING A LENGTH APPROXIMATELY EQUAL TO THE LENGTH OF THE RISER PIPE AND A DIAMETER APPROACHING THE DIAMETER OF THE RISER PIPE, AND BEING POSITIONED WITHIN SAID RISER PIPE, THE BLADE BEING RELATIVELY NARROW WITH RESPECT TO THE DIAMETER OF THE RISER PIPE THEREBY DIFINING AN OPEN SPACE WITHIN SAID HELIX, SAID HELICAL BLADE BEING ATTACHED TO A SHAFT CONCENTRIC WITH AND PROJECTING FROM THE TOP OF THE RISER PIPE, THE BLADE AND SHAFT BEING ROTATABLE, AND VERTICALLY MOVABLE IN A LIMITED DISTANCE, MOTOR MEANS OPERATIVELY CONNECTED TO THE SHAFT FOR ROTATION THE SHAFT AND HELICAL BLADE, AND MEANS SYNCHRONIZED WITH SAID MOTOR MEANS FOR IMPARTING A VERTICAL RECIPROCATING MOTION TO THE SHAFT AND HELICAL BLADE AT LEAST ONCE IN EVERY REVOLUTION, THE UPWARD PORTION OF THE RECIPROCATING MOVEMENT BEING AT A LOW AND SUBSTANTIALLY UNIFORM RATE, THE DOWNWARD PORTION BEING ABRUPT AND ACCELERATED, THEREBY DISLODGING CALCIUM PARTICLES FROM THE RISER PIPE AND PREVENTING ADHESION OF SAID CALCIUM PARTICLES TO THE HELICAL BLADE. 